All of your kitties are just beautiful and you take fabulous pictures too! I'm an aspiring photographer myself.....just bought a Nikon D70.
I was in the same situation as you. I had two cats initially that I've had for years, when my oldest daughter brought home a pregnant stray. It was rough going for awhile as my other female cat is very dominant and doesn't like anyone or anything that takes my attention off of her.
When Abby had her kittens, I had to keep them all locked up in my bedroom for 8 weeks because I was worried that Shelby would hurt them. She would sit at my door and hiss and growl because she could smell them and knew they were in there. I made the introductions when the kittens were 8 weeks old and it went surprisingly well. I guess by then, my other two cats were used to the smell even though they hadn't been formally introduced. I also think that by this point, Abby was tired of being cooped up in the bedroom and had become a little less protective of the kittens.
When Shelby came face to face with the kittens, she hissed at both of them......but it was like a "I'm the boss around here and don't you forget it" type of hiss and she never tried to hurt them. In fact, Shelby practically took over the kittens and claimed them as her own. (she was fixed at young age and never had any kittens) Abby was such a young mother that she didn't seem to mind Shelby taking over and doing all the "work."
Abby and Shelby have continued to have their problems and I was planning to find Abby another home. I just had her spayed last week though and the girls seem to be getting along better now. Fortunately, in our case, Abby only had two kittens, (Thomas and Cooper) so it was an easy decision to keep them both. We have become a 5 cat household and it's really not as difficult as it seems. There's just more photo opportunities and more laughs and love to go around.
Hopefully, once Pepper has her kittens, things will settle down a bit. Please post pictures of the babies once they arrive!! I can't wait to see them.